1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a lithium secondary battery which can prevent deformation of an external shape of a battery due to swelling under a high-temperature condition and can prevent deterioration in performance of a battery, and a method for manufacturing thereof.
2. Description of the Related Art
Lithium batteries capable of charging and discharging, in general, are being vigorously studied along with the development of portable electronic devices such as cellular phones, notebook-type computers or camcorders. There are various kinds of secondary batteries, including nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni-MH) batteries, lithium ion batteries, lithium polymer batteries, lithium-metal secondary batteries, zinc-air storage batteries and so on. Specifically, lithium secondary batteries are rapidly developing because of their longlasting lifetime characteristics, that is, approximately 3 times the lifetime characteristics of nickel-cadmium (Ni—Cd) batteries or nickel-metal hydride (Ni-MH) batteries used to supply power to electronic devices, and their excellent energy densities per unit weight. Lithium secondary batteries can be classified into liquid electrolyte batteries and solid electrolyte batteries according to the kind of electrolyte used. In general, a battery using a liquid electrolyte is referred to as a lithium-ion battery, and a battery using a polymeric electrolyte is referred to as a lithium polymer battery.
In the manufacture of a lithium secondary battery, a positive electrode current collector and a negative electrode current collector are first coated with a material prepared by mixing each electrode active material, a binder and a plasticizer, to form a positive and a negative electrode sheet. Then, the positive and the negative electrode sheet are stacked or wound on both sides of a separator to form a positive-negative electrode assembly (jelly-roll) having a predetermined shape, followed by inserting the jelly-roll into a battery case, thereby completing a battery.
In the case of a lithium secondary battery prepared by employing a volatile electrolytic solution, the internal pressure of the battery abnormally increases due to an increase in volume of a closed battery and evaporation of an electrolytic solution under a high-temperature condition of 60° C. (Celsius) or higher, resulting in the swelling of the battery. Thus, the battery undergoes deformation in external shape. Also, electrode sheets of the battery are adversely affected in adhesiveness, which lowers the utilization efficiency of the electrode sheets during an electrochemical reaction, resulting in deterioration in battery performance. It is known that the following factors give rise to swelling of a lithium secondary battery occurring when the battery is exposed to a high-temperature condition: (1) a vigorous reaction between active materials and an electrolytic solution at high temperature; (2) an increase in vapor pressure due to evaporation of an electrolytic solution at high temperature; and (3) the content of moisture in a battery.
Korean registered patent publication No. 10-274097 for Method for Manufacturing a Lithium Ion Polymer Battery discloses removal of impurity-containing air by evacuating a battery pack before injecting an electrolytic solution into the battery pack, which is performed as only a part of injecting the electrolytic solution into the battery back.
As described above, a pressure reducing step has not been conventionally adopted. Otherwise, the pressure reduction step has been partially adopted in the step of injecting an electrolytic solution or sealing followed by an initial formation process of a battery, or in the step of degassing after the initial formation process. However, a pressure reducing effect is not noticeable. The pressure reducing effect is only enough to confirm by the naked eye that the pressure is subatmospheric. Thus, problems with the prior art, including volume expansion of a battery kept at high temperature or deterioration in battery performance, cannot be overcome.